1. Field of the Invention
This invention relates to a circuit and method for universally interfacing a sensor, regardless of whether current sourcing or current sinking, with a data bus.
2. Description of Related Art
Conventional current sinking sensors vary current from a current source, typically as a function of a sensed parameter. Conventional current sourcing sensors vary a current supply, also as a function of a sensed parameter.
Certain conventional interface circuits, such as those used with programmable controllers, have a mechanical selector switch for choosing between operating modes of an interface circuit or to select either a current sinking input or a current sourcing input.
U.S. Pat. No. 4,689,499 discloses an electronic circuit having two levels of comparators and associated circuitry that define two threshold levels between which neither comparator is actuated. In response to current flowing from or into a sensor terminal, a sensor input terminal has an input current that varies outside of a window of the threshold levels. Comparator output terminals are each connected to a separate output circuit which produces a signal that can be transmitted on a data bus, in response to actuation of either comparator.
Many conventional interface circuits have no capability to universally accept inputs from either type of sensor, a current sourcing sensor or a current sinking sensor. Other conventional interface circuits can indiscriminately accept inputs from either a current sourcing sensor or a current sinking sensor, but many conventional interface circuits require multiple levels or subcircuits, which increases the cost for a universal interface.
There is apparent need for a simple and low cost interface circuit that can produce an output signal or no output signal as a function of an input current from either a current sourcing sensor or a current sinking sensor.